The method described herein relates generally to an in-situ method for sealing fluid cooled conduits. More specifically, the method relates to an in-situ method for sealing brazed joints in fluid cooled conduits for a generator.
It is known that the windings of a dynamoelectric machine stator can be more effectively cooled by causing a dielectric fluid such as deionized water to flow through the windings inside the main insulation, such as in hollow strands of a multi-strand conductor bar. In a dynamoelectric machine stator winding, usually more than one of these insulated conductor bars lie in each slot formed in the laminated stator core. Very often, two such bars are employed, the top or radially inner bar in the slot being subjected to greater ohmic losses, and hence greater generation of heat, than the bottom or radially outer bar. It has also been known that the temperature difference between top and bottom bar can be reduced by the use of a two-pass system where the fluid flows through the length of the machine in a top bar and then returns through the machine in a bottom bar. Thus, the coolest fluid flows through the top bar with its greater heat losses and, after the temperature of the fluid has been raised somewhat, it returns through a bottom bar which has less heat losses. In this manner, the temperature difference between the top and bottom bars is reduced.
In a two-pass system, the fluid pressure drop in the restricted flow passages can result in large pumping losses in a large machine. Thus it has also been known to use a single-pass system wherein the fluid supplied at a series loop, electrically connecting top and bottom bars at one end of the machine, flows through top and bottom bars in parallel and is collected at the series loop at the other end of the machine, to be re-cooled and recirculated. However, with this arrangement, since fluid of the same temperature is supplied to both top and bottom bars, the top bar will be at a greater average temperature than the bottom bars. Therefore, changes in load on the machine, and the start-up and shutdown cycles, can cause relative movement between the bars in a slot due to differential thermal expansion and contraction, which causes abrasion and damage to the insulation.
In large generators, the windings are such that the terminating ends or phase leads of a group of connected coils forming a phase winding are disposed at circumferentially spaced locations about the periphery at one end of the core. The connections are such that a top bar can be electrically connected with a bottom bar of the same phase, located approximately 120 electrical degrees away, by means of an arcuate conductor called a connection ring. The connection ring is also electrically connected through lower leads to bushings leading through the casing. For a three-phase generator, there would ordinarily be six such connection rings, six lower leads, and six bushings disposed on one end of the generator. The connection rings and the lower leads carry substantial currents and must also be cooled. This also may be done by cooling internal passages with a fluid.
The hollow conductors external to the stator include the phase leads, series loops and connection rings. Typically, deionized water is passed through these hollow conductors/conduits. During manufacture of the phase leads, series loops and connection rings many brazed joints are required to connect the various conductors/conduits and fittings. These brazed joints often contain phosphorus, and the combination of phosphorous and water may result in corrosion and subsequently leaks. It will be appreciated that water leaking in or around a utility scale generator is not desired. If the brazed joints are failing or leaks are detected, one option is to completely replace all the phase leads, series loops and connection rings. Unfortunately, this approach is very expensive and time consuming. New parts will have to be purchased, which can take many months to procure and manufacture, and these parts may take days or even a week to install.